1. Field of the Invention
The invention relates to the structure of security elements for documents, to devices for testing documents incorporating such elements, and to a method of testing the genuineness or authenticity of such documents.
2. The Prior Art
Heretofore, the authenticity of documents incorporating optically diffractive security elements has usually been checked by complex optical test procedures. For instance, testing of documents incorporating an optically diffractive security element or so-called OVD (optically variable device) has been impossible within document handling machines in view of their very high running speeds. German patent specification 27 47 156 discloses a method and a test instrument for testing for counterfeits of holographically secured identity cards. The OVD is reproduced and then visually examined. Such a procedure is unsuited for fast, efficient and automated tests. European patent specification 0,042,946 discloses a device for the production of scanning patterns which are tested by a laser, mirror and line systems as well as by a photo detector. The device is expensive and of low cost efficiency. It would be even more expensive if the material were to be tested without prior sorting. In order to avoid presorting processes it would be necessary to provide a multiple counterfeit test system or to conduct repetitive tests.
European patent specification 0,092,691 A1 discloses an apparatus for detecting security strips in bank notes. The material-specific absorption bands of a plastic safety strip are measured by two channels of light transmitted in the infrared range and at wave lengths of about 5 μm. A counterfeit or quality test utilizing optically diffractive security elements which provide for metallic reflection such as reflex holograms or kinegrams, is neither taught by the European patent specification and nor could it be performed by the disclosed apparatus. From British patent specification 21 60 644 A a reflected light procedure of examining bank notes by means of a line scan camera is known, and from Swiss patent 652,355 it is known to examine cards which are provided with a special layer structure by reflected or transmitted light. In either case the examination depends on comparing the information of an image against originals. In either version, reflections and marks of use pose problems and constitute a great disadvantage. An automatic counterfeit examination of information contained in a hologram is disclosed by German Offenlegungsschrift 38 11 905. The information incorporated in a hologram is examined and analyzed by a transmitter and a receiver which are mounted directly opposite each other. However, the opposite disposition of transmitter and receiver leads to disadvantageous overmodulation and, in some cases, to damage of the receiver because of directly impinging light between successive bank notes. Moreover, the examination of used bank notes is rendered virtually impossible because of accidental reflections caused by creases.
In each case, the known examination processes require exact positioning of the documents and are often unsuited for high-speed processing.
German patent specification 196 04 856 A1 proposes the examination of condition, quality and registration mark of optical safety features provided on documents such as, for instance, bank notes, as metallically reflecting layers such as kinegrams, holograms and the like. A metallically reflecting security feature incorporated in the document is scanned by transmitted light by at least one electronic camera, preferably a CCD line scan camera, and the actual values thus derived are then compared against desired values by known image evaluation processes so that defective bank notes may be marked or used notes may be separated by a sorting device. The apparatus described in German patent specification 196 04 856 A1 is characterized by a known transport device for moving a document in the area of the electronic camera, an infrared radiation source on the side of the document opposite the camera and by the optical axis of the camera enclosing an angle other than 180° relative to the optical axis of the radiation source, and by the transport device being preferably constructed of transport belts separated from each other laterally of the transport direction. This apparatus or method also suffers from the drawback that used bank notes in particular those with creases caused by use and bank notes which have a damaged or surface-soiled kinegram are not recognized as being authentic bank notes. Moreover, while the described method and appurtenant apparatus are automated, they are unsuited nevertheless for use with high-speed bank note machines running at rates of 1,200 pieces per minute.
Security features depending on optical diffraction or OVD on documents such as, for instance, German 100 and 200 Mark bank notes, are at present manually or visually examined for damage, registration accuracy, exact marginal instance, etc. Examination is performed visually during bank note production as well as during possible sorting out of bank notes which are to be withdrawn from circulation. Such procedure is time-consuming and cost-inefficient.
German patent specification DE 195 42 995 A1 discloses, among other things, a method of testing the authenticity of a data carrier by comparing the different available data. The patent offers the following possibilities:                Comparing the standard image of the hologram against a stored image;        Comparing the data of the hologram against data in a defined area of the data support and/or those in a storage unit;        Comparing the data of the hologram against data made available by an input unit;        Comparing the individual image of the hologram against data of the input unit of the storage unit and/or against data of the defined area.        
Also, dyes with special physical properties are known as security elements for rendering documents and bank notes secure. A distinction can be made between dyes which are recognizable either visually or tactily, and those which can only be recognized by special equipment, depending upon the given physical properties of the dye, e.g., electrical conductivity or fluorescence. Interference dyes belong to the group of dyes recognizable without special equipment. These may be found in German Mark notes of the 1996 et seq series (issued in 1997). Changes in the viewing angle result in a change in color. This tilting effect permits rapid and uncomplicated manual individual examinations. Colors with fluorescent or magnetic properties, or of a specific electrical conductivity, can only be detected by appropriate equipment. Current testing devices have a low resolution, however, which necessitates security elements of large dimensions in order to ensure an acceptable recognition.
The examination of printing dyes of different conductivities has been found to be disadvantageous as different conductivities have to be examined successively by different testing devices in the same testing operation or by the same testing device with appropriately configured software in two test operations. Moreover, the measuring accuracy is low when the conductivity of the test field is low. Testing electrically conductive printing dyes which are of different conductivities because of their applied thickness and because of the different electrical conductivities of substrates of the marks, is not possible by any known testing devices because of their low resolution.
The known characteristics, test zones and test structures to be examined, as well as the methods and devices for testing the authenticity of objects, security documents, especially bank notes, suffer from the main disadvantage which is inherent in their being known, i.e. known to an extent which enables a counterfeiter on the basis of his knowledge of the testing methods and devices and their function to draw conclusions in respect of the characteristics to be examined, the testing zones and structures. This compels devising a completely novel task for examining objects, security documents, especially bank notes, the solution to which must be precipitated in a novel system of the use of test characteristics, testing methods and devices, in order to prevent easy detection of information codes and the copying thereof.